In a conventional flat battery of this type, the end of a conductive piece extending from the lateral side of the first electrode plate (located at the center of the battery) is inserted into a notch formed in an insulating plate located beneath the inner surface of the first outer case and is pressed against or welded to the first outer case, thereby being connected thereto. Thus, in such a conventional flat battery, the collector mechanism connecting the first electrode plate to the first outer case is complicated, and a short-circuit might arise in the battery if the insulating plate is shifted.